Windows 8's uptake was stuck in reverse for the second straight quarter as the reputation-challenged operating system fell behind the pace set by Windows Vista six years ago, according to data released Friday.

Web metrics firm Net Applications' figures for July put the combined user share of Windows 8 and 8.1 at 12.5 percent of the world's desktop and notebook systems, a small drop of six-hundredths of a percentage point from June. That decline was atop a one-tenth-point fall the month before, the first time the OS had lost user share since its October 2012 debut.

Windows 8 accounted for 13.6 percent of the personal computers running Microsoft's Windows. The difference between the numbers for all personal computers and only those running Windows was due to Windows powering 91.7 percent of all personal computers, not 100 percent.

While in June Windows 8's user share came dangerously close to the sluggish uptake tempo of Windows Vista, in July Windows 8's pace fell below Vista's for the first time. (Computerworld erred in calling Windows 8's uptake slower than Vista's in the early stages of the former's lifespan based on incorrect comparisons.)

At the point in Vista's post-release timeline that corresponded to July, the 2007 operating system ran on 13.6 percent of all personal computers -- a larger percentage than Windows 8's last month -- and on 14.3 percent of all Windows PCs. The latter is the most credible, as it accounts for the slightly-greater dominance of Windows at the time. (When Vista was in its 21st month after launch, Windows powered 94.9 percent of all personal computers.)

That Windows 8's uptake performance has not matched Vista's is important because the latter, widely panned at the time, has earned a reputation as one of Microsoft's biggest OS failures. By association, then, Windows 8 looks to be the same.

While Windows 8 again lost user share in July, Windows 7 gained another seven-tenths of a percentage point to close the month with 51.2 percent. It was the fifth straight month that the 2009 operating system has grown its share. The surge has not been surprising, since most industry analysts have said that the recent uptick in computer sales has been due to businesses replacing the now-retired Windows XP with Windows 7.

Windows 7 has grown by nearly twice the amount of Windows 8 in the past six months.

Windows XP's user share fell half a percentage point in July, accounting for 24.8 percent of all personal computers, and 27.1 percent of only those running Windows. The decline came after a month where the aged OS remained flat. In the last six months, XP has contracted by 4.4 points.

Computerworld now projects that Windows XP will still be running between 20 percent and 22 percent of the world's personal computers at the end of 2014.

Another analytics company, Ireland's StatCounter, had different numbers for Windows. StatCounter's figures are typically at odds with those from Net Applications because they measure with dissimilar methodologies: StatCounter tallies "usage share" by counting page views to show how active users of each OS are on the Web, while Net Applications estimates "user share" by collating unique visitors, which more closely resembles user base than does StatCounter's data.